Recent advances in the field of semiconductor technology have led to the development of chip carrier packages. Chip carrier packages are generally comprised of a planar housing, having a semiconductor chip therein. Conductive members are provided on the chip carrier for bonding to metallized areas on the surface of a printed circuit board. In addition to chip carrier packages, other electronic components, such as resistors and capacitors, have been developed for mounting directly on the surface of a printed circuit board. The term surface mounted component is commonly used to generically describe any type of electronic component which is adapted for mounting on the surface of a printed circuit board.
The technique now in common use for mounting such components on the surface of a printed circuit board comprises the steps of: (a) applying a controlled amount of a bonding material, typically solder paste, on the metallized areas on the circuit board; (b) placing the component on the surface of the circuit board such that the conductive members thereof are in contact with the solder paste-coated metallized areas; and (c) heating the solder paste to bond the conductive members of the component to the metallized areas.
There are various methods by which solder paste may be deposited on the metallized pads on a printed circuit board. One approach is to use the dispensing tool described in U.S. Pat. No. 4,515,297 issued to us on May 7, 1985 and assigned to the instant assignee. The dispensing tool described in the aforesaid patent comprises a chamber into which solder paste is drawn via a vacuum. Once the chamber is filled, a piston is placed within the chamber on top of the solder paste. A pressure is applied against the piston to force the solder paste out of the chamber through a nozzle having a plurality of apertures each in registration with a separate one of the metallized areas on the printed circuit board. In order to dispense a different pattern of solder paste, the nozzle must be replaced with one having the proper arrangement of apertures, which is a time consuming process.
Solder paste can also be deposited (printed) onto metallized areas on the printed circuit board by forcing the paste through openings in a substrate (e.g., a stencil) placed in intimate contact with the circuit board. The stencil has openings therein arranged in the same pattern as metallized areas on the circuit board, so that only the metallized areas are exposed through the stencil openings. Solder paste is applied to the stencil and a squeegee blade is moved across the surface thereof to force the paste into the stencil openings and onto the metallized areas on the printed circuit board. All of the metallized areas exposed through the stencil openings can be advantageously coated with solder paste during one or two passes of the squeegee blade across the stencil.
High quality printing of solder paste onto the printed circuit board requires a consistent volume of paste to be uniformly distributed across the leading edge of the squeegee blade and a minimal volume to be distributed across the trailing edge of the blade. In the past, prior art dispensing apparatus have incorporated only one squeegee blade. After a single pass of the blade across the stencil, a quantity of paste will typically remain proximate the leading edge of the blade. Before making the next pass in the opposite direction, the blade is made to hop or jump over this quantity of paste so the paste is now distributed along the leading edge of the blade. Unfortunately, solder paste has a high viscosity and tends to stick to the blade. Thus, paste will often cling to what now becomes the trailing edge of the blade once the blade is displaced in the opposite direction during a second pass across the stencil. The result is streaking of solder paste on the stencil and an uneven deposit of solder paste on the circuit board.
To a lesser degree, the disadvantages attendant in printing of solder paste onto a substrate using a single squeegee blade are also present in printing other less viscous materials. In an effort to improve the printing of less viscous conductive inks and the like on a printed circuit board, U.S. Pat. No. 3,464,351 issued on Sept. 2, 1969 to E. G. DeHart et al. discloses an apparatus embodying a pair of squeegee blades which serve to contain the material to be printed therebetween. In practice, the material is supplied into the region between the blades from a remote source under pressure. While the DeHart et al. apparatus may be suited for materials having a moderate viscosity, substantial difficulties are likely to arise in attempting to use the apparatus to print materials having a high viscosity, such as solder paste. In order to force solder paste from a remote source into the region between the blades of DeHart et al., a very high pressure is required which can cause the solder paste to separate. Further, when the pressure is removed, there will be a long lag time before the paste forced from the region between the blades stops flowing.
Accordingly, there is a need for a technique for dispensing a highly viscous material, such as solder paste, onto a substrate.